Maternal melatonin readily crosses the placenta to provide the fetus with time-of-day and day length information. To determine if melatonin could have a broader role during development than is currently recognized, melatonin receptor expression was examined in somatic sites during Siberian hamster embryogenesis. Using 125I-labeled-2-iodomelatonin ([125I]MEL), melatonin receptor expression was examined in whole fetuses by in vitro autoradiography. [125I]MEL binding sites were first apparent at gestational day (GD) 10 over the primitive oral pharynx. From GD 12 to 14, binding was present over the nasal pharynx, Rathke's pouch, caudal arteries, and over the thyroid gland during its migration along the thyroglossal duct. By GD 16, Rathke's pouch had differentiated into the pituitary gland, which continued to express specific [125I]MEL binding until birth. From GD 16 until birth, binding was no longer detectable over the thyroid gland, but persisted over the nasal epithelium. At all ages, binding sites exhibited high affinity for [125I]MEL and appeared to be coupled with guanosine nucleotide-binding proteins. These data suggest that melatonin receptors are expressed in several somatic sites, including Rathke's pouch and the thyroid gland, during fetal development.